A large-scale high-precision optical system destined for use and operation in the outer space has to be assembled, adjusted and evaluated on the earth or ground before being located in the outer space. In general, for the adjustment of the high-precision optical system on the ground, image forming performance or capability of the optical system concerned is measured, whereon the optical system is adjusted on the basis of the data acquired through the measurement.
A method of inspecting the high-precision optical system such as a telescope is described comprehensively, for example, in an article titled “TESTS OF OPTICAL SYSTEMS, (1) METHOD BASED ON INTERFERENCE” contained in the literature “0 plus E”, No. 143, pp. 109–113. According to this known method, a collimated light beam is caused to impinge onto an optical system under test, whereon wavefront error ascribable to aberrations occurring internally of the optical system under test is measured.
As the causes or factors for the wavefront error, there can be mentioned the following.    (1) Wavefront aberration ascribable to deviation in the optical disposition of optical elements or optics in the optical system under test (hereinafter referred to as the optical system deviation or optic deviation),    (2) wavefront aberration ascribable to polishing error of optical elements or optics incorporated in the optical system under test,    (3) wavefront aberration ascribable to fluctuation of the atmosphere,    (4) wavefront aberration ascribable to gravitational deformation of the optical elements incorporated in the optical system under test, and    (5) wavefront aberration ascribable to vibration of the optical system under test and the measuring apparatus.
Of the causes or factors for the wavefront error mentioned above, the wavefront aberration (2) of high order is suppressed to a possible minimum because the optical elements are ordinarily polished to the highest degree of precision. However, the wavefront aberrations (3), (4) and (5) mentioned above make appearance because of the very measurement or test performed on the ground. Accordingly, in the case where the optic disposition is adjusted such that the measured value of the wavefront error becomes extremely small without compensating for the influences of the wavefront aberrations (3), (4) and (5), the optic deviation will take place in the optical system and the wavefront error will become remarkable as compared with the optical system which suffers no deviation, when the optical system is used or operated in the outer space.
Further, it is noted that a light source of low luminance such as e.g. an electric lamp has heretofore been employed as the light source in most of the practical applications. Consequently, image data have to be acquired by resorting to an extended-time exposure with the number of data reading times being correspondingly decreased, while minimizing the dark current of the image pickup device by cooling with a view to realizing the signal-to-noise ratio (SNR) demanded for the processing.
In general, the wavefront aberration ascribable to the fluctuation of the atmosphere is considered to vary or change temporally at random. Accordingly, the influence of the fluctuation of the atmosphere can certainly be mitigated by taking advantage of the time-integration effect which inherently accompanies the extended-time exposure. In that case, however, a lot of time is taken for each measurement, and it is difficult or impossible to analyze the time-based change rate and the period of change of the wavefront aberration ascribable to the fluctuation of the atmosphere within the time period set for the measurement.
For the reason mentioned above, optimization of the measurement environment for the fluctuation of the atmosphere on the basis of the measured data and reduction of the integration time in dependence on the measurement environment could not be realized. In particular, in the case where the non-interferometric type wavefront measuring apparatus is employed for the alignment adjustment, it is required to repeat the measurement and the adjustment at as high a rate as possible while optimizing the environment for the measurement.
On the other hand, when the source of low luminance (electric light bulb) is employed as the light source, a pin hole is used for collimating the light rays emitted from the source, and thus the light rays exiting the pin hole constitute the light source, as is known in the art. With this arrangement, the utilization efficiency of luminous energy of the light source is degraded, necessitating it to extend further the exposure time duration. Besides, it is necessary to install the light source in the main housing of the non-interferometric type wavefront measuring apparatus or in the close vicinity thereof, providing an obstacle for implementation of the main housing in small size and light weight. In addition, the influence of heat generation of the light source can not be neglected, because the wavefront aberrations are caused to change due to expansion and contraction of the main housing and the individual optical elements or optics and under application of stress, riving rise to a problem.
Furthermore, for effecting the extended-time exposure, the influence of the dark current in the image pickup device must be suppressed to a minimum. Such being the circumstances, the cooling arrangement is usually adopted particularly when the semiconductor image pickup device is employed.
However, incorporation of the cooling unit in the image pickup device will naturally result in increasing of size and weight of the main housing portion of the non-interferometric type wavefront measuring apparatus, providing an obstacle to the attempt for miniaturization of the non-interferometric type wavefront measuring apparatus. In order to cope with this problem, it is necessary to use the light source of high luminance and dispose it separately from the main housing of the non-interferometric type wavefront measuring apparatus.
By the way, in the field of the astronomical observation, there has arisen in recent years a demand for improvement of the image forming performance or image quality of the optical system. In reality, in conjunction with the reflecting telescope for the astronomical observation, efforts are being made for developing and realizing a reflecting mirror of large aperture and ideal geometrical precision with a view to making available the high-definition images of dark celestial bodies.
As a technical matter which exerts remarkable influence to the image forming performance or the image quality of the large-scale reflecting telescope such as mentioned above, there can be mentioned a so-called assembling adjustment error. More specifically, in order to ensure the prescribed image forming performance or image quality, it is necessary that deviation of relative positional relation between the reflecting mirrors falls within a predetermined range. Parenthetically, the deviation mentioned just above is referred to as the assembling adjustment error. By way of example, in the Gregory-type reflecting telescope disclosed in Tuneta-Saku's “Solar-B VISIBLE LIGHT TELESCOPE” contained in “KOKURITSU TENMONDAI NEWS (National Astronomical Observatory News)”, No. 91, January 2001, the assembling adjustment error must not exceed 10 μm at the least in the arrangement in which a primary mirror having the aperture of 500 mm and a secondary mirror of a smaller size are disposed with a distance of 1500 mm therebetween. Such high degree of precision is of course difficult to realize with the assembling process which is not accompanied with adjustment. Accordingly, as a means for coping with this problem, there is adopted a method of inspecting the image forming performance of the reflecting telescope and then adjustment is so performed that the image forming performance of the reflecting telescope lies within a predetermined range.
As a method of visibly displaying the measured wavefront error, there are generally adopted a contour plotting method such as illustrated in FIG. 33 and a three-dimensional plotting method such as illustrated in FIG. 34.
Further, there is a method of displaying by plotting mode-based wavefront error components such as comma aberration, astigmatism, etc. by mathematically fitting the wavefront error to an appropriate function.
In this conjunction, it is noted that as the deviation of the relative positional relation between the primary mirror and the secondary mirror in the reflecting telescope from the design value thereof becomes greater, the plotted value of the wavefront error mentioned above become more remarkable. By repeating the assembling adjustment of the primary mirror and the secondary mirror as well as the inspection or test concerning the wavefront error described above, it is certainly possible to reduce the wavefront error to within the predetermined range.
For solving the problem mentioned above, it is required to eliminate the wavefront aberrations making appearance due to the measurement on the earth from the measured data to thereby extract only the wavefront error ascribable to the so-called optical system deviation or optic deviation and carry out the adjustment process until the wavefront error ascribable to the optic deviation becomes zero.
In particular, in the adjustment of the optic disposition with high accuracy, the wavefront error components ascribable to the factors or aberrations (3), (4) and (5) mentioned previously make appearance relatively remarkably. Thus, it is required that the wavefront measuring apparatus can ensure sufficiently large dynamic range and high measurement resolution.
Among the wavefront error factors, the aberrations (3) and (5) change temporally at random. Accordingly, it is possible to reduce the wavefront error ascribable to these factors or aberrations by resorting to a time-average processing. In that case, the wavefront error components other than those ascribable to the factors (3) and (5) can be measured with the dynamic range being enlarged.
However, in the interference measuring method adopted in the inspection of high-precision mirror surface, the wavefront error is ordinarily determined on the basis of the interference fringe produced between a reference wavefront and the measured wavefront. Consequently, the time-average processing executed upon formation of the interference fringe results in disappearance of the interference fringe.
For mathematically integrating the interference fringes, it is required to acquire once the interference fringe images and perform the summation average arithmetic operation on the images after correctively compensating for the interference fringe shifts brought about by the vibration and the fluctuation of the atmosphere. To this end, there is demanded a processing system which is capable of measuring massive interference fringe images, estimating the fringe correcting quantities and effectuating the fringe correction before summation. Besides, lots of time must be assigned to the fringe correction processing in each measurement.
As will now become apparent from the foregoing, in order to shorten the time taken for the measurement and analysis, such wavefront measuring means is demanded which is capable of correctively canceling out the wavefront error components ascribable to the factors or aberrations (3), (4) and (5) from the measured wavefront and executing the processing of extracting from the measurement data only the component ascribable to the optic disposition deviation within a short time. Further, severe requirement is imposed that magnitude of optical system deviation, i.e., optic disposition deviation, and direction thereof be estimated from the extracted wavefront error quantity to thereby adjust the reflecting telescope so that the optics or optical elements thereof assume the optimal optical disposition in the zero-gravity state.
It is further noted that according to the optical system assembling and adjusting method of the prior art, operator is required to estimate the assembling adjustment error of the optical system from the visibly indicated or plotted values of the wavefront error as described above. However, the components of the wavefront error plotted reflect the factors which depend on the environmental conditions such as temperature change, fluctuation of the atmosphere, deformation under the dead load and the like in addition to the assembling adjustment error. Consequently, there has arisen the problem that the assembling adjustment accuracy undergoes degradation in dependence on the environmental conditions.
Moreover, the assembling adjustment error contains a plurality of parameters concerning position, angle, direction and magnitude. However, difficulty is encountered in determining discriminatively these parameters on the basis of the visibly indicated or plotted wavefront error. Under the circumstances, it is necessary to repeat the adjustment and the test a large number of times, which of course means remarkable degradation in the process efficiency.
With the preset invention which has been made with a view to solving the problems mentioned above, it is contemplated as an object thereof to provide an optical system deviation estimating apparatus, an optical system deviation adjusting apparatus, an optical system deviation estimating method, and an optical system deviation adjusting method which are capable of correctively adjusting only the wavefront error ascribable to the optical system deviation and capable of measuring or inspecting the optical system destined for use in the zero-gravity environment without any appreciable difficulty on the earth.
Further, it is an object of the present invention to provide an optical system deviation estimating apparatus which can carry out inspection or test as to the assembling adjustment error of the optical system with high accuracy and at high rate without undergoing the influence of environmental conditions and which can indicate the assembling adjustment error visibly intelligibly.